Aromatic diblock copolymers for lubricant and concentrate compositions and methods thereof

ABSTRACT

A lubricant composition comprises (A) a major amount of an oil of lubricating viscosity and (B) a minor amount of a diblock copolymer comprising a poly(monovinyl aromatic hydrocarbon) block and a hydrogenated poly(conjugated diene) block where the diblock copolymer has a weight average molecular weight of 10,000 to 50,000 and a poly(monovinyl aromatic hydrocarbon) content of 5 to 45% by weight. A concentrate composition, a method for improving viscosity performance of a lubricant composition for an internal combustion engine, and a method for lubricating an internal combustion engine comprise the diblock copolymer which is especially useful for reducing soot-induced viscosity increase in a compression-ignited internal combustion engine equipped with an exhaust gas recirculation system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention involves lubricant and concentrate compositions and methods thereof that comprise a diblock copolymer. The compositions and methods of this invention are especially useful for improving viscosity performance in an internal combustion engine.

2. Description of the Related Art

Compression-ignited internal combustion engines, also termed diesel engines, of the heavy duty type can use exhaust gas recirculation (EGR) in efforts to reduce engine exhaust emissions of substances, such as nitrogen oxides, that are being regulated for health and environmental concerns. Among the consequences of recirculating the exhaust gas through the engine are different soot structures and increased viscosity of the oil or lubricant composition of the engine at lower soot levels compared with engines without EGR. It is desirable that the oil or lubricant composition exhibit minimal viscosity increase such as for example less than 12 mm²/sec (cSt) at a soot loading of 6%.

A lubricant composition that has less change in its viscosity with a change in temperature is said to have a high viscosity index. A viscosity modifier, usually a polymeric material, can be incorporated into a lubricant composition to increase the viscosity index of the lubricant composition and to improve its rheological properties and/or lubricating performance including viscosity performance.

Anderson in U.S. Pat. No. 4,036,910 discloses block copolymers as viscosity index improvers for lubrication oils.

Elliott in U.S. Pat. No. 4,073,737 discloses that hydrogenated copolymers of conjugated dienes and when desired a vinyl aromatic monomer are useful as oil additives.

Sutherland in U.S. Pat. No. 6,083,888 discloses dispersant viscosity index improvers from a linear diblock copolymer or a radial copolymer wherein N-vinyl imidazole groups are grafted onto the copolymer.

Wedlock et al. in U.S. Pat. No. 6,303,550 and Research Disclosure Journal No. 38644 disclose a lubricating oil composition comprising a di-block copolymer of poly(monovinyl aromatic hydrocarbon) and hydrogenated poly(conjugated diene) as a dispersant additive.

Ritchie et al. in U.S. Pat. No. 6,715,473 disclose that soot induced kinematic viscosity increase of lubricating oil compositions for heavy duty diesel engines equipped with EGR systems operating in a condensing mode can be ameliorated by selection of viscosity modifier, lubricating oil flow improvers, detergents and dispersants.

Dispersant viscosity index improvers can function in a lubricating oil such as an internal combustion engine oil to improve both viscosity and dispersant performance since they are polymeric materials that contain a polar or dispersing group such as e. g. a polystyrene-hydrogenated polyisoprene diblock copolymer to which is grafted N-vinylimidazole as disclosed in U.S. Pat. No. 6,083,888. High molecular weight diblock copolymers containing an aromatic block can function in an internal combustion engine lubricant oil to provide both viscosity and dispersancy performance such as e. g. diblock copolymers of polystyrene and hydrogenated polyisoprene that have a total molecular weight of 100,000 or 135,000 as disclosed in U.S. Pat. No. 6,303,550. A further disclosure in U.S. Pat. No. 6,303,550 is that low molecular weight diblock copolymers, containing a critical chain length of an aromatic block and preferably 60 weight % or greater of the aromatic block, can also provide dispersancy performance such as e. g. a polystyrene-hydrogenated polyisoprene diblock copolymer having a total molecular weight of 22,000; a molecular weight of 17,380 for the polystyrene block; and a 79% by weight polystyrene content.

The present invention provides a lubricant composition comprising a low molecular weight diblock copolymer having a low aromatic content where the diblock copolymer functions to provide performance as a viscosity modifier and unexpectedly as an effective dispersant in engine tests. This performance includes increasing the viscosity index and increasing the soot dispersion, which reduces undesirable soot-induced viscosity increase, of the lubricant in an internal combustion engine, especially a diesel engine that uses EGR. The dispersant performance of the diblock copolymer of this invention is unexpected since the diblock copolymers of earlier disclosures having dispersant performance are considerably higher in total molecular weight or have a considerably higher aromatic content.

SUMMARY OF THE INVENTION

An object of the present invention is to provide lubricant and concentrate compositions that are useful in functional fluids.

A further object of the invention is to provide lubricant and concentrate compositions that are useful in an internal combustion engine oil.

A still further object of the invention is to provide a lubricant and concentrate compositions that are useful in a compression-ignited internal combustion engine oil

An additional object of this invention is to provide lubricant and concentrate compositions that are useful in a compression-ignited internal combustion engine oil where the engine is equipped with an exhaust gas recirculation system.

Additional objects and advantages of the present invention will be set forth in the Detailed Description which follows and, in part, will be obvious from the Detailed Description or may be learned by the practice of the invention. The objects and advantages of the invention may be realized by means of the instrumentalities and combinations pointed out in the appended claims.

To achieve the foregoing objects in accordance with the present invention as described and claimed in this application, a lubricant composition comprises (A) a major amount of an oil of lubricating viscosity, and (B) a minor amount of a diblock copolymer comprising a poly(monovinyl aromatic hydrocarbon) block and a hydrogenated poly(conjugated diene) block wherein the diblock copolymer has a weight average molecular weight of 10,000 to 50,000 and a poly(monovinyl aromatic hydrocarbon) content of 5 to 45% by weight.

In an embodiment of the invention a concentrate composition comprises a concentrate-forming amount of an oil of lubricating viscosity, and the above described diblock copolymer of component (B).

In another embodiment of this invention a method for improving the viscosity performance of a lubricant composition for an internal combustion engine comprises incorporating into the lubricant composition a viscosity performance-improving amount of the above described diblock copolymer of component (B).

In a further embodiment of the invention a method for lubricating an internal combustion engine comprises supplying to the engine the above described lubricant composition comprising components (A) an oil of lubricating viscosity and (B) a diblock copolymer.

DETAILED DESCRIPTION OF THE INVENTION

The present invention comprises lubricant and concentrate compositions and methods thereof comprising a low molecular weight diblock copolymer of low aromatic content where the compositions and methods are useful in or as functional fluids to include for example an internal combustion engine oil lubricant, a power transmission fluid, an automotive or industrial gear oil, a farm tractor hydraulic fluid, a hydraulic fluid, a normally liquid fuel composition, a grease, and a metalworking fluid. In several embodiments of the invention the lubricant composition or concentrate composition are for an internal combustion engine, a spark-ignited internal combustion engine, a compression-ignited internal combustion engine, or a compression-ignited internal combustion engine having an exhaust gas recirculation system.

The invention can comprise a lubricant composition comprising (A) a major amount of an oil of lubricating viscosity, and (B) a minor amount of a diblock copolymer comprising a poly(monovinyl aromatic hydrocarbon) block and a hydrogenated poly(conjugated diene) block wherein the diblock copolymer has a weight average molecular weight of 10,000 to 50,000 and a poly(monovinyl aromatic hydrocarbon) content of 5 to 45% by weight

Oil of Lubricating Viscosity

The oil of lubricating viscosity of component (A) can comprise a natural oil, a synthetic oil, or a mixture thereof. The oil of lubricating viscosity can have a kinematic viscosity in mm²/s (or centistokes) at 100° C. of 2-70, and in other instances of 3-50, or 4-40. Natural oils can comprise crude and refined mineral oils derived from petroleum, coal, shale, or a mixture thereof; animal oils; plant or vegetable oils; or a mixture thereof. Synthetic oils can comprise both unhydrogenated and hydrogenated polyolefins, carboxylic acid esters prepared from mono- and/or polycarboxylic acids or reactive equivalents thereof and from mono- and/or polyhydric alcohols, alkylated aromatics, polyglycols and derivatives thereof, phosphate esters, silicone oils, hydrocarbons prepared by a gas to liquid process such as for example a Fischer-Trospsch process, or a mixture thereof. The oil of lubricating viscosity can include the American Petroleum Institute Group I, II, III, IV and V base oils. The oil of lubricating viscosity can be present in the lubricant composition of the present invention in a major amount, and in other embodiments can be present in the lubricant composition on a weight basis at greater than 50%, at 60 to 99.9%, at 70 to 99.5%, or at 80 to 95%.

Diblock Copolymer

The copolymer of component (B) of this invention can comprise a polymer from at least one monovinyl aromatic hydrocarbon monomer and at least one conjugated diene monomer. The copolymer can comprise a) a random copolymer formed by polymerization of a mixture of at least one monovinyl aromatic hydrocarbon and at least one conjugated diene, b) a block copolymer formed by sequential polymerization of the at least one monovinyl aromatic hydrocarbon and the at least one conjugated diene, or c) a mixture thereof. The block copolymer can comprise one or more blocks of at least one monovinyl aromatic hydrocarbon and one or more blocks of at least one conjugated diene. In an embodiment of the invention the copolymer of component (B) comprises a diblock copolymer comprising a poly(vinyl aromatic hydrocarbon) block and a hydrogenated poly(conjugated diene) block. The vinyl aromatic hydrocarbon of the poly(vinyl aromatic hydrocarbon) block can be a single monomer or can be a mixture of two or more monomers. The conjugated diene of the poly(conjugated diene) block can be a single monomer or a mixture of two or more monomers. In another embodiment of the invention the diblock copolymer of component (B) is derived from a single monovinyl aromatic hydrocarbon monomer and a single conjugated diene monomer.

The monovinyl aromatic hydrocarbon monomer of the copolymer of component (B) can comprise any vinyl-substituted aromatic hydrocarbon, and in other instances can comprise vinyl-substituted aromatic hydrocarbons having 8 to 18 carbon atoms, or 8 to 13 carbon atoms. The monovinyl aromatic hydrocarbon can comprise for example styrene; an alkyl-substituted styrene such as for example alpha-methylstyrene, 3- and/or 4-methylstyrene, alpha,2-dimethylstyrene, and 2,4-dimethylstyrene; an alkoxy-substituted styrene such as for example 2-, 3- and 4-vinylanisole and beta-methoxystyrene; a vinylnaphthalene such as for example 1- and 2-vinylnaphthalene; an alkyl-substituted vinylnaphthalene; or a mixture thereof. The monovinyl aromatic hydrocarbon can have one or more alkyl and/or alkoxy substituents, and the alkyl and/or alkoxy substituents can contain 1 or more carbon atoms and usually contain 1 to 6 carbon atoms. In an embodiment of the invention the monovinyl aromatic hydrocarbon is selected from the group consisting of styrene, an alkyl-substituted styrene, an alkoxy-substituted styrene, and a mixture thereof. In another embodiment of the invention the monovinyl aromatic hydrocarbon is selected from the group consisting of styrene, alpha-methylstyrene, a vinylanisole, and a mixture thereof.

The conjugated diene monomer of the copolymer of component (B) can comprise any conjugated diene, and in other instances can comprise a conjugated diene having 4 to 24 carbon atoms, 4 to 8 carbon atoms, 4 to 8 carbon atoms, or 4 to 6 carbon atoms. The conjugated diene can comprise for example a butadiene such as for example 1,3-butadiene and isoprene; a pentadiene such as piperylene (usually a mixture of cis and trans isomers) and 4-methyl-1,3-pentadiene; a hexadiene such as 1,3- and 2,4-hexadiene and 2,5-dimethyl-2,4-hexadiene; an octadiene such as 1,3-octadiene; or a mixture thereof. In an embodiment of the invention the conjugated diene is selected from the group consisting of 1,3-butadiene, isoprene, piperylene, and a mixture thereof. In another embodiment of the invention the monovinyl aromatic hydrocarbon is styrene and the conjugated diene is isoprene or 1,3-butadiene or a mixture thereof.

The monovinyl aromatic hydrocarbon and conjugated diene monomers of this invention are available from numerous manufacturers and chemical suppliers. Both the random copolymer and block copolymer can be prepared by well known procedures including by an anionic living polymerization using an alkali metal or alkali metal compound as a catalyst such as for example an organo alkali metal like a butyllithium. While the random copolymer is formed by polymerizing a mixture of the monomers, a block copolymer is formed by polymerizing a first monomer or monomers such as for example styrene to form a first polymer block followed by polymerization onto the first polymer block of a second monomer or monomers such as isoprene to form a second polymer block. The polymerization can be run at −20 to 120° C., and in other instances at 0 to 100° C., or at 20 to 80° C. The polymerization can be run in the presence of an inert solvent to include a nonpolar solvent such as a hydrocarbon like cyclohexane or a mixed nonpolar/polar solvent such as a mixture of a hydrocarbon and an ether. The random or block copolymer can then be selectively hydrogenated to reduce olefinic unsaturation by well known methods such as hydrogenation using a reduced nickel on kieselguhr catalyst. The hydrogenation can reduce at least 90% of the olefinic unsaturation, and in other instances at least 95% or at least 98% of the olefinic unsaturation can be reduced. U.S. Pat. No. 3,554,911 describes procedures for preparing a copolymer using an anionic catalyst and for selectively hydrogenating the copolymer.

The diblock copolymer of component (B) of this invention can have a weight average molecular weight, based on gel permeation chromatography measurements using a polystyrene standard, of 10,000 to 50,000, and in other embodiments can have a weight average molecular weight of 12,000 to 47,000; 14,000 to 44,000; 10,000 to 25,000; or 25,000 to 50,000. The diblock copolymer of component (B) can have a poly(monovinyl aromatic hydrocarbon) content on a weight basis of 5 to 55%, and in other embodiments can have a poly(monovinyl aromatic hydrocarbon) content of 5 to 45%, 16 to 43%, or 27 to 41%. The diblock copolymer of component (B) can be present in a lubricant composition of this invention in a minor amount, and in other embodiments can be present in the lubricant composition on a weight basis at less than 50%, at 0.1 to 20%, at 0.1 to 10%, at 0.2 to 6%, or at 0.2 to 3%.

Other Additives

The lubricant composition of the present invention can further comprise (C) at least one other additive. The other additive or additives are normally selected based on what type of functional fluid use the lubricant composition is intended for. Additives generally useful in various functional fluids, to include lubricant compositions for an internal combustion engine, can comprise at least one additive selected from the group consisting of a dispersant; an antiwear agent; an antioxidant; a detergent; a corrosion inhibitor to include for example an alkenylsuccinic acid, tolyltriazole or a derivative thereof, and dimercaptothiadiazole or a derivative thereof; an antifoam agent to include for example a silicone oil and a polyacrylate; a pour point depressant to include for example an esterified copolymer of maleic anhydride and styrene, and a polymethacrylate; a viscosity modifier that is different from component (B); a friction modifier to include for example a vegetable oil, an ester and amide of a fatty acid such as glycerol monooleate and oleamide, and an organo molybdenum compound; and an extreme pressure agent to include for example a polysulfide of an olefin, and a phosphate ester or derivative thereof. In an embodiment of the invention the lubricant composition further comprises (C) at least one other additive selected from the group consisting of a dispersant, an antiwear agent, an antioxidant, a detergent, a corrosion inhibitor, an antifoam agent, a pour point depressant, and a viscosity modifier that is different from component (B).

The detergent can comprise a neutral or basic metal salt, usually where the metal is an alkali or alkaline earth metal, of an alkylarylsulfonic acid; an alkylphenol, a formaldehyde coupled alkylphenol, a sulfur coupled alkylphenol, or a mixture thereof; an aliphatic or aromatic carboxylic acid having 12 or more carbon atoms; or a mixture thereof.

The dispersant can comprise a hydrocarbyl-substituted succinimide and/or a hydrocarbyl-substituted succinate ester where the hydrocarbyl group has a number average molecular weight of 700 to 3,000 and the succinimide or succinate is prepared from a polyamine, to include polyethylenepolyamines, and/or a mono- or polyhydric alcohol, to include pentaerythritol; a Mannich base; a hydrocarbyl-substituted amine derived from a hydrocarbyl group having a number average molecular weight of 700 to 3,000; or a mixture thereof. The hydrocarbyl group of the dispersant is a univalent group that is predominately hydrocarbon in nature but it can contain heteroatoms, to include oxygen and halogens, in the main carbon chain or in chains or groups attached to the main carbon chain. In an embodiment of the invention the dispersant comprises a reaction product of a hydrocarbyl-substituted succinic acylating agent and a polyethylenepolyamine wherein the hydrocarbyl substituent is derived from a polyisobutylene, and the dispersant reaction product has a nitrogen to carbonyl ratio that is greater than 1. The polyisobutylene of the dispersant reaction product can have a number average molecular weight of 700 to 3,000; 900 to 2,500; or 1,300 to 2,500. The number of succinic acylating groups per hydrocarbyl group in the dispersant reaction product can be 0.8 to 2, 1.1 to 2, or 1.5 to 2. The succinic acylating group of the dispersant reaction product can be a succinic anhydride group or a reactive equivalent thereof to include a succinic acid group or succinate ester group. The dispersant reaction product can have a TBN, total base number expressed in milligram equivalents of KOH per gram of sample, of 25 to 70, 30 to 65, or 35 to 60. The dispersant to include the above described dispersant reaction product can be present in the lubricant composition on a weight basis at I to 12%, at 2 to 10%, or at 2 to 8%.

The viscosity modifier is generally a polymeric material and can comprise a viscosity index improver, a dispersant viscosity modifier that usually contains a polar group incorporated either via polymerization of a monomer or by grafting and that can improve both the viscosity index and dispersancy of a lubricant composition, or a mixture thereof. The viscosity modifier can comprise an olefin homopolymer such as a polyisobutylene; an olefin copolymer such as an ethylene-propylene copolymer or ethylene-propylene-diene copolymer; an acrylate homopolymer or copolymer; a methacrylate homopolymer or copolymer such as a copolymer of 2 or more methacrylate esters from 2 or more alcohols differing in carbon number and/or branching; a maleic anhydride-styrene copolymer or a derivative thereof such as an amine salt of a partially esterified copolymer of maleic anhydride and styrene; a hydrogenated styrene-diene copolymer such as a copolymer of styrene and 1,3-butadiene; or a mixture thereof. The dispersant viscosity modifier can comprise functionalized forms of the above described viscosity modifier that through the functionalization contain polar groups to include for example basic amino groups such as a diphenylamine group that can provide, in addition to viscosity performance, dispersant and/or antioxidation performance. The functionalization can be done by including a polar monomer in the polymerization to form the viscosity modifier, by grafting a polar reagent onto the viscosity modifier, by reaction of a polar reagent with a reactive group on the viscosity modifier, or a mixture thereof. In an embodiment of the invention component (C) comprises a viscosity modifier other than component (B), a dispersant viscosity modifier, or a mixture thereof.

The antioxidant can comprise at least one member selected from the group consisting of a diarylamine to include an alkylated diphenylamine; a hindered phenol to include a base catalyzed addition product of 2,6-di-t-butylphenol and an alkyl acrylate ester; a sulfurized olefinic compound to, include a sulfurized olefin, a sulfurized olefin containing carboxylate ester, a sulfurized olefin containing triglyceride, or a mixture thereof.

The antiwear agent can comprise an organic phosphate to include a metal salt of a dialkyl dithiophosphate; a sulfurized olefinic compound as described above under the antioxidant; a dithiocarbamate to include a molybdenum dithiocarbamate; or a mixture thereof. In several embodiments of the invention the antiwear agent can comprise a metal salt of a dialkyl dithiophosphate where the dialkyl dithiophosphate is derived from one or more primary alcohols, from one or more secondary alcohols, or from a mixture of one or more primary alcohols and one or more secondary alcohols.

The above described additives of component (C) are well known and are available commercially and/or can be prepared by known procedures. U.S. Pat. No. 4,904,401 discloses several of the additives of component (C) to include procedures for their preparation. Each of the additives of component (C) can be present in the lubricant composition on a weight basis at 0.001 to 14%; and in other instances at 0.001 to 11%, or at 0.001 to 8%.

In several embodiments of this invention the lubricant composition can have on a weight basis a sulfated ash content of greater than 1.3%, 1.3% or less, 1.2% or less, or 1.1% or less; can have on a weight basis a sulfur content of greater than 0.5%, 0.5% or less, 0.4% or less, or 0.3% or less; and can have on a weight basis a phosphorus content of greater than 0.14%, 0.14% or less, 0.13% or less, or 0.12% or less.

Concentrate Compositions

The lubricant additives of components (B) and (C) can be added directly to a lubricant composition of this invention, however they are normally combined with component (A), an oil of lubricating of lubricating viscosity, to form one or more concentrate compositions. A concentrate composition provides a concentrated form of the lubricant composition that has a higher lubricant additive content. A concentrate composition is handleable and transferable, more economical to ship, and readily blended with additional oil and optionally with additional additives, additional concentrates, or a mixture thereof to form a lubricant composition. In an embodiment of the invention a concentrate composition comprises a concentrate-forming amount of an oil of lubricating viscosity of component (A) and a diblock copolymer of component (B). In another embodiment of the invention the concentrate composition can further comprise at least one other additive of component (C). The concentrate composition can have a concentrate-forming amount of an oil of lubricating viscosity, and in other instances can have an amount of an oil of lubricating viscosity on a weight basis of 15 to 75%, 25 to 65%, 35 to 55%, or 55 to 95%. The concentrate composition can have an amount of component (B) or components (B) and (C) on a weight basis of 25 to 85%, 35 to 75%, 45 to 65%, or 5 to 45%.

Preparation of Compositions

The lubricant composition of the present invention can be prepared by admixing the components. In several instances of the invention the lubricant composition can be prepared by admixing components (A) and (B) or by further admixing component (C) with components (A) and (B). In several other instances of the invention the lubricant composition can be prepared by admixing component (A) and a concentrate composition comprising component (B) or by further admixing one or more additives of component (C), one or more concentrate compositions comprising component (C), or a mixture thereof with component (A) and a concentrate composition comprising component (B). The lubricant and concentrate compositions of the invention are usually prepared by admixing the components with a mixing device from ambient to an elevated temperature of 100° C., 90° C., or 80° C.

Method for Improving Performance

In an embodiment of the invention a method for improving the viscosity performance of a lubricant composition for an internal combustion engine comprises incorporating into the lubricant composition a viscosity performance-improving amount of the diblock copolymer of component (B) where the lubricant composition comprises a major amount of an oil of lubricating viscosity of component (A). The performance-improving amount of the diblock copolymer of component (B) can be an amount as described above in paragraph [0027]. The improvement in viscosity performance of a lubricant composition for an internal combustion engine can comprise a reduction in soot-induced viscosity increase, an increase in the viscosity index, or a combination thereof. The internal combustion engine can be a compression-ignited or spark-ignited engine. The compression-ignited engine can be equipped with an exhaust gas recirculation system. In another embodiment of the invention the method for improving the viscosity performance of a lubricant composition for an internal combustion engine comprises a Mack™ T-11 diesel engine having an exhaust gas recirculation system and the improvement in viscosity performance is a prevention or reduction in soot-induced viscosity increase. In a further embodiment of the invention a nitrogen-containing dispersant is partially replaced with a diblock copolymer of component (B) to improve viscosity performance of a lubricant composition for an internal combustion engine which further improves lubricant performance by reducing corrosion of metal bearings and/or increases durability of seals.

Method for Lubricating

In another embodiment of this invention a method for lubricating an internal combustion engine comprises supplying to the engine a lubricant composition comprising a major amount of an oil of lubricating viscosity of component (A) and a minor amount of a diblock copolymer of component (B) as described above in paragraphs [0022] through [0027]. The lubricant composition can further comprise one or more other additives of component (C) as described above. The internal combustion engine can be a compression-ignited or spark-ignited engine. The compression-ignited engine can be equipped with an exhaust gas recirculation system.

The following examples are provided to illustrate the invention, but are not intended to and should not be used to limit the scope of the invention.

Mack™ T-11 EGR Test

Four lubricant compositions are prepared and run for 252 hours in the Mack™ T-11 engine test that uses exhaust gas recirculation to evaluate the ability of the lubricant to reduce soot-induced viscosity increase. During each test run samples of the lubricant are removed at 12 hour intervals, and the % by weight soot and kinematic viscosity at 100° C. are measured. Test results are reported as the amount of soot (% soot at kinematic viscosity increase or % soot @ kv incr) present in the lubricant when the viscosity (kinematic viscosity at 100° C.) of the lubricant increases significantly. A lubricant is improved in reducing soot-induced viscosity increase when it contains a greater amount of soot before its viscosity increases significantly compared to another lubricant. Example No. Composition¹, wt % % Soot @ KV Incr⁶ 1 (comparative) 3.6% Dispt + 0.75% VM² 4.0 2 (comparative) 3.6% Dispt + 0.85% VM³ 6.4 3 (comparative) 4.7% Dispt + 0.78% VM⁴ 5.0 4 4.7% Dispt + 1.47% VM⁵ 6.4

While the engine oil of comparative Example 2 containing a high molecular weight styrene-isoprene diblock copolymer reduces soot-induced viscosity increase compared to the baseline oil of comparative Example 1, the engine oil of invention Example 4 containing a low molecular weight, low styrene content styrene-isoprene diblock copolymer unexpectedly reduces soot-induced viscosity increase compared to the baseline oil of comparative Example 3. This improvement in viscosity performance is unexpected because prior disclosures teach that improved viscosity performance for soot-induced viscosity increase is obtained from diblock copolymers having a much higher molecular wt. or a much higher aromatic content compared to the invention diblock copolymer. Furthermore, this reduction in soot-induced viscosity increase by the invention diblock copolymer is unexpected because (1) the styrene block becomes more oil soluble having less tendency to adsorb on the surface of soot particles as its molecular weight decreases and (2) the isoprene block is less able to provide steric repulsion to prevent soot agglomeration and consequent viscosity increase as its molecular weight decreases. For these reasons, low molecular weight styrene-isoprene diblock copolymers would be expected to suspend or disperse soot poorly.

Each of the documents referred to in this Detailed Description of the Invention section is incorporated herein by reference. All numerical quantities in this application used to describe or claim the present invention are understood to be modified by the word “about” except for the examples or where explicitly indicated otherwise. All chemical treatments or contents throughout this application regarding the present invention are understood to be as actives unless indicated otherwise even though solvents or diluents may be present. 

1. A lubricant composition, comprising; (A) a major amount of an oil of lubricating viscosity; and (B) a minor amount of a diblock copolymer comprising a poly(monovinyl aromatic hydrocarbon) block and a hydrogenated poly(conjugated diene) block wherein the diblock copolymer has a weight average molecular weight of 10,000 to 50,000 and a poly(monovinyl aromatic hydrocarbon) content of 5 to 45% by weight.
 2. The lubricant composition of claim 1 wherein the monovinyl aromatic hydrocarbon is selected from the group consisting of styrene, an alkyl-substituted styrene, an alkoxy-substituted styrene, and a mixture thereof.
 3. The lubricant composition of claim 1 wherein the conjugated diene is selected from the group consisting of 1,3-butadiene; isoprene; piperylene; and a mixture thereof.
 4. The lubricant composition of claim 1 wherein the monovinyl aromatic hydrocarbon is styrene and the conjugated diene is isoprene or 1,3-butadiene.
 5. The lubricant composition of claim 1 wherein the diblock copolymer has a weight average molecular weight of 12,000 to 47,000 and a poly(monovinyl aromatic hydrocarbon) content of 16 to 43% by weight.
 6. The lubricant composition of claim 1, further comprising: (C) at least one other additive selected from the group consisting of a dispersant, an antiwear agent, an antioxidant, a detergent, a corrosion inhibitor, an antifoam agent, a pour point depressant, and a viscosity modifier that is different from component (B).
 7. The lubricant composition of claim 6 wherein the lubricant composition is prepared by admixing the components.
 8. The lubricant composition of claim 1 further comprising a dispersant wherein the dispersant comprises a reaction product of a hydrocarbyl-substituted succinic acylating agent and a polyethylenepolyamine wherein the hydrocarbyl substituent is derived from a polyisobutylene, and the dispersant reaction product has a nitrogen to carbonyl ratio that is greater than
 1. 9. The lubricant composition of claim 1 further comprising an antiwear agent wherein the antiwear agent comprises a metal salt of a dialkyl dithiophosphate wherein the dialkyl dithiophosphate is derived from one or more secondary alcohols.
 10. The lubricant composition of claim 1 further comprising an antioxidant wherein the antioxidant comprises at least one member selected from the group consisting of a hindered phenol, a diarylamine, and a sulfurized olefinic compound.
 11. The lubricant composition of claim 1 further comprising a viscosity modifer wherein the viscosity modifier comprises an ethylene-propylene copolymer.
 12. The lubricant composition of claim 1 wherein the lubricant composition has on a weight basis a sulfated ash content of 1.1% or less, a sulfur content of 0.5% or less, and a phosphorus content of 0.12% or less.
 13. A concentrate composition, comprising: a concentrate-forming amount of an oil of lubricating viscosity; and component (B) of claim
 1. 14. A method for improving the viscosity performance of a lubricant composition for an internal combustion engine, comprising: incorporating into the lubricant composition a viscosity performance-improving amount of component (B) of claim 1 wherein the lubricant composition comprises component (A).
 15. The method of claim 14 wherein the engine is a compression-ignited internal combustion engine.
 16. The method of claim 15 wherein the engine is equipped with an exhaust gas recirculation system.
 17. A method for lubricating an internal combustion engine, comprising; supplying to the engine the lubricant composition of claim
 1. 18. The method of claim 17 wherein the engine is a compression-ignited internal combustion engine. 